In manufacturing thin-film semiconductor devices using organic semiconductor thin films, such as organic thin-film transistors (organic TFT), the organic semiconductor thin films can be formed by applying a coating/printing process. For example, an organic semiconductor thin film composed of an organic semiconductor material with a single configuration can be obtained by coating a substrate with a solution prepared by dissolving the organic semiconductor material in a solvent and then drying the resulting coating. Therefore, attempts can be made to increase the substrate size and decrease the manufacturing cost as compared with conventional semiconductor devices using inorganic semiconductor materials such as silicon (Si) and the like. In addition, the coating/printing process or the like has a low process temperature and is thus capable of forming on plastic substrates, and the thin-film semiconductor devices are expected as semiconductor devices having flexibility. As an example of this, it has been reported that a back plane is prepared by providing organic TFT on a plastic substrate, and a flat-panel display device such as a liquid crystal display device, an OLED display device, or the like is manufactured using the back plane.
As materials constituting organic semiconductor thin films, for example, a wide range of materials, such as polythiophene, pentacene, rubrene, and the like, have been investigated, and it has been reported that some of the thin-film semiconductor devices have mobility equivalent to or higher than that of thin-film semiconductor devices using semiconductor thin films composed of amorphous silicon (for example, refer to “Applied Physics Letters”, {HYPERLINK “http://scitation.aip.org/dbt/dbt.jsp?KEY=APPLAB&Volume=69”, Volume 69}, Issue 26, 1996, p. 4108-4110).
However, thin-film semiconductor devices using organic semiconductor thin films have the problem that mobility is degraded by heating. In an actual experiment, it was confirmed that a mobility of 0.14 cm2/Vs before heating becomes a mobility of 6×10−4 cm2/Vs in a state of heating to 180° C. in a nitrogen atmosphere, and the mobility is degraded to 1/100 or less by heating. Since organic semiconductor materials are not oxidized because of heating in a nitrogen atmosphere, and the organic semiconductor materials used are not thermally decomposed even by heating to 180° C., it is found that the deterioration of mobility due to heating is not caused by changes in properties of the organic semiconductor materials.
Accordingly, an object of the present invention is to provide a method for forming a semiconductor thin film, which is capable of suppressing decrease in mobility due to heating and characteristic deterioration due to the decrease in mobility and which is capable of forming a semiconductor thin film with improved heat resistance according to simple procedures, and provide a method for manufacturing a thin-film semiconductor device by applying the forming method.